Current-Controlled Alternating Steady State Free Precession for Rapid Conductivity Mapping

Introduction: High resolution cross-sectional conductivity (σ) distribution of biological tissues can be measured with magnetic resonance electrical impedance tomography (MREIT) (1), wherein the magnetic flux density (Bz) induced by current injection through a pair of electrodes synchronized with a MR pulse sequence appears as perturbation in image phase, and then the relation between ∇Bz and ∇σ is exploited to generate conductivity images (2). In this work, we develop a current-controlled, alternating steady state free precession (SSFP) based MREIT for rapid conductivity imaging as an alternative to conventional time-consuming spin echo (SE) based MREIT. In the proposed SSFP MREIT, increasing population of capacitive current-induced higher-order echoes potentially deteriorates phase information, while coherent accumulations of current-encoded spin magnetizations result in nonlinear relation between image phase and Bz. To reduce capacitive current during radio-frequency (RF) pulses while retaining high phase sensitivity and signal-to-noise ratio (SNR) in Bz, four different current injection types are investigated using the signal formulation derived from the Bloch equation. Bz is extracted from SSFP MREIT data by solving nonlinear regularized inverse problem. Phantom experiments demonstrate that the proposed method, if compared with conventional SE MREIT, achieves rapid conductivity mapping without apparent loss of accuracy.